Serendip is an independent site partnering with faculty at multiple colleges and universities around the world. Happy exploring!

Synchronized Cycles

SerendipUpdate's picture

Biology 103
2000 Second Web Report
On Serendip

Synchronized Cycles

Jeanne Braha

Last spring, a good friend of mine moved into the area. Since then, we've spent quite a bit of time together, both at work and just for fun. Some people say I acquire her more southern speech patterns when I'm around her more often. Maybe, but I can say for sure that one thing that has changed is that our menstrual cycles have slowly moved towards each others, so now we are almost perfectly synchronized. Women have most likely observed this phenomenon since the first time anyone needed to borrow a pad or tampon. However, the first formal investigation in biological circles was not done until 1970. (1) Martha McClintock, just finished her junior year at Wellesley, was attending a conference where it was mentioned that rats in captivity together tended to ovulate simultaneously. McClintock noted that she had observed this pattern amongst her dormmates. Suddenly, she had a senior thesis topic and wound up publishing her results in Nature during her first year of graduate school.

McClintock's work attracted much more attention than the average undergraduate research project because of its implications: she postulated that pheromones, airborne chemical signals released by humans from skin glands into the environment that have an affect on the physiology or behavior of other members of the same species, were the catalyst for the synchronization of ovulatory cycles. Her initial hypothesis was that roommates' cycles would have the highest correlation, because they had the strongest shared air space. However, the studies she did at Wellesley showed that good friends had the closest timing, after 3 to 4 months together.

Subsequent studies of rats in captivity validated the airspace concept: those living together tended to synchronize, but those isolated did not. Rats in separate cages but with a tube to connect the air between the cages also had the same degree of synchronization.(4) Computer modeling followed, but McClintock needed to do more studies of humans to further support the hypothesis that pheromones were the mechanism for the synchronization. As a professor of biopsychology at the University of Chicago, she did just that. Nature once again published her findings (2, with coauthor K. Stern).

Stern and McClintock recruited women to participate and told them all that they were in the control population. The women wore gauze pads in their armpits with a few drops of alcohol to prevent bacterial growth. The pads were then wiped on the upper lips of other women. (You can see why they all had to believe they were control subjects) When the contents of pads worn by women in their follicular phase were wiped under noses, the receptive women generally had shortened cycles, i.e. they ovulated sooner. Those receiving ovulatory phase signals experienced longer cycles. The conclusion was that pheromones excreted in the armpits had a regulatory (not explicitly synchronizing, though that is the most commonly identified result) effect on women who were receptive to and exposed to the pheromones. The composition of the pheromone was not identified, but the idea that a pheromone was at work received more support.

McClintock's critics picked up on this gap. Biochemists tried to fill that void by looking for a specific chemical that could act as the pheromone (3). Instead of trying to influence women's cycles to match up, they worked the other way: they identified women who had synchronized cycles and searched them for a substance. Those women whose cycles matched up had a higher olfactory acuity (they were better able to smell) to the odor of 3-androstenol, a steroid. This protein is synthesized by microbiological modification of odorless substances originally present in apocrine secretions. Levels of the steroid in women's armpits are highest prior to ovulation. The next step would be to do a lip-wipe study with only 3-androstenol on the gauze pads, to more accurately test its affects.

Given that pheromones are sent out by women at different phases in their ovulatory cycle, and that most women are receptive (to some degree - variation was noted in receptivity (3)) to those chemicals, the next step is to describe how the pheromone influences the timing of the cycle.

Just as genes do not cause diseases, pheromones do not directly cause behaviors. Jim Kohl (6) explains that pheromones influence gene expression primarily in nerve cells that secrete gonadotropin-releasing hormone (GnRH). GnRH has a regulatory effect on luteinizing hormone and follicular stimulating hormone, both of which are associated with parts of the menstrual cycle. Some of Jim's supporting evidence was a little iffy - he suggests that pheromones are indicated in coitus-induced ovulations, though I suspect that there may be mechanical explanations of this as well (7). However, the basic mechanism he suggests seems sound. The vomeronasal organ, proteins very similar to smell receptors and in the same nasal region, but encoded by separate genes, detects the pheromones and sends a signal to the hypothalamus. The pheromones are called primer pheromones (10), for their role in priming the endocrine system and to distinguish them from identification hormones that most mammals (think dogs and cats) use to mark their territory. This triggers it to send GnRH to the pituitary gland. The pituitary then sends out corresponding pulses of follicular-stimulating hormone and luteinizing, which correspond to the production of estrogen and progesterone in females, which have direct bearing on the menstrual cycle.

Along with determining the precise mechanism by which the process occurs, McClintock has searched for an explanation of why women's cycles would synchronize. Human women certainly appreciate the convenience of having the same needs as their friends, but the same can not be said for sure of mice. McClintock (4) suggests that these signals would have been indispensable to women in societies with more limited resources. In close contact with other women, the pheromones could be an unconscious aid prior to the onset of puberty, during lactation, or at other critical life phases. Signals either shortening or lengthening the cycle would suggest whether it were "safe" (whether sufficient resources were available) to begin one's maturation. If not, pheromones could potentially "prevent" the beginning of one's menses. During lactation, ovulation is not particularly common, because of the amount of energy required to properly nourish another being. However, it is known to happen (my sister is just under 15 months older than me - my mother must have ovulated when Lizzy was 6 months old and still breastfeeding). Another pregnancy would be a major drain on a mother and her newborn if there were not enough food to sustain the trio. McClintock again (4). cited the rats: when in the wild or in a laboratory-controlled burrow setting, rat puppies are over twice as likely to survive when their litter was one of many in the colony. Pups of mothers not in synch with her population have about a one in three chance of survival.

While most humans now live in environments with enough resources on hand to eliminate the above concerns, the effect of pheromones and an understanding of the system are still proving useful. GnRH is already being capitalized on by "the Pill", aka oral contraceptives. By inhibiting GnRH production, the mid-cycle surge of luteinizing hormone that stimulates the release of an egg from the ovary is prevented. Research is currently exploring ways to use knowledge of pheromones and their effects on ovulatory cycles in infertility treatments. Other uses are slightly less noble: in Japan, underwear laced with pheromones is available to give folks an advantage in attracting a potential mate. (6)

Moreover, investigations into the realities of pheromones are significant in the way that they is happening, and the mere fact that they are happening at all. The story of McClintock's thesis and her subsequent career is inspiration for all non-world-experts. Perhaps the empirical approach being taken to this subject will destigmatize it to some extent. Discussion of these processes, which happen on a regular basis for the better part of most women's lives are taboo in most conversations, though the topics of nutrition, aging or seasonal allergies are fair game. Finally, I believe that a wider understanding of pheromones and the ways in which humans are constantly in communication will help begin to break down our current highly individualistic schemas. Until we realize how interdependent we are, effecting great ecological and social change will be terribly difficult. I hope that pheromones become a much more common topic in biology, and not just as it relates to ants or territorial puppies. More accurate pinpointing of the chemicals involved and how different levels of sensitivity are determined might help inspire confidence in the findings. Until then, I think there is a fortune to be made in some form of over-the-counter pheromone-based pill that would allow women to manipulate their ovulatory schedule to better fit their calendars.

WWW Sources

1)Menstrual Odor, an excerpt from a book about McClintock's (and Hillary Clinton's) at Wellesley and a very fuzzy copy of the original Nature article

2)New Evidence Shows that Pheromones Influence Menstrual Cycle, an excerpt from a book about McClintock's (and Hillary Clinton's) at Wellesley and a very fuzzy copy of the original Nature article

3)ChemSenses Online, Go to page 407 of volume 25 in 2000. 4)Radio National: Health Report with Norman Swan Radio interview with Martha McClintock. 5)Human Pheromones and the Neuroendocrinology of Behavior, The text of a presentation given by Jim Kohl. 6) The Scent of Eros , Jim Kohl's "online resource", and lots of advertisement for his book, The Scent of Eros

7) Woman: An Intimate Geography by Natalie Angier. This is a real book, not a website, but I recommend it as a non-science-y narrative about the way our bodies work.

8)Menstrual Convergence, Jen Oelberger's WOMB paper from 1996

9)Diagram of Pheromones and Hormonal Feedback, accompanies the text of Jim Kohl's site and presentation. Fairly easy to follow

10)Hormones of the Reproductive System, The link provided on the class outline by Dr. Grobstein.

11)Pheromones, from the same general site as #10.


Comments made prior to 2007

It is my experience that synchronized menstrual cycles are not based on pheromones. I always believe in that theory until the last few years. I frequently make female friends over the internet. The same phenomenon occurs online as well. I can not explain it. All I can say is that it disproves the pheromone theory. I am a professional woman in the scientific industry. If I could see how this would make me $$ I'd spend more time studying ... Reader on the web, 8 March 2006


Anonymous's picture

synchronus mensus

Everywhere that I have ever worked this has happened.

Most extreme case...with an age range of early 20's to pre-menopause
In a department of 15 women - 8 of us synch'd (we worked together the most & sat within arms reach of each other.
Same department - 3 of us miscarried within days of each other
Same department - 6 of us pregnant with a 5 month spread in due dates -- 5 healthy singles & 1 set of twins

Anonymous's picture

synchronized cycle with web friend

I have a dear friend whom I met over the internet. We have been good friends for 2.5 yrs. Our lives have been very closely aligned, getting sick around the same time, children going through similar sleep cycles, marriages going through similar phsases, etc. During the duration of our friendship, I had and IUD in with no period up until this July. At which point my first period came some 40 days after the removal of the IUD. Which landed on her cycle, but we didn't realize it until the next month, when we both got sick within days of eachother, and started our periods the same day. The next cycle mine came a day before hers/ The next cycle, we landed on the same day yet again, with my cycle being 30 days long and hers 29.

Now I find this odd, because she is on the pill which is supposed to regulate her periods, and me - before I had an IUD my cycles were usually in the 25-28 day window, but occassionally landed in the 30 day window. I find it odd that our cycles have synchronized so quickly since I got off my IUD, and we are only internet friends and have never met in person.

Anonymous's picture

Anyone, who doesn't believe this occurance, just listen to me!!

My sister's and I all share the same menstrual cycle, give or take a few days, and for many years, my best friend and I had the same cycle, but we gradually grew apart, but we still make jokes about it, but now ours are different.